This invention relates to the production of hollow spherical shells of material and, more particularly, to a method for producing uniform hollow shells of a liquid which have particular application for use as targets in laser fusion.
Controlled thermonuclear fusion holds promise of being one of the solutions to long term energy shortage problems. One problem in releasing fusion energy is to confine an intense hot plasma, for example plasma of hydrogen isotopes deuterium-tritium, long enough for fusion reactions to take place. Techniques of inertial confinement are currently being investigated as an answer to this problem. This calls for rapidly compressing fuel material to high densities. One technique, called laser fusion, utilizes high powered lasers to implode small fuel pellets, for example pellets of hydrogen isotopes deuterium-tritium, to thousands of times their normal liquid density and thereby initiate a fusion reaction. The laser light is focused onto a low density atmosphere of material as it evaporates from the surface of the pellet. This light is absorbed in the plasma atmosphere by electron-ion collisions or by plasma instabilities. In both cases the hot electrons travel in toward the center of the pellet and heat the pellet surface. The surface cools itself by ablation, that is by rapidly expelling material. This material travelling outward creates an equal and opposite force which compresses the pellet. The compression continues until the pressure created is greater than the ablation force.
One of the problems encountered in laser fusion is that plasma instabilities due to using high powered lasers tend to create extremely energetic electrons. These electrons can penetrate the core of the fuel pellet prematurely, thus making compression more difficult; a phenomenon called "preheat". These electrons also have a long range and therefore reduce the heat transfer between the pellet surface and the plasma atmosphere which results in uneven heating of the pellet, known as "decoupling".
The threshold for plasma instabilities can be increased by using hollow fuel pellets. In such case, the ablation pressure acts for a longer time and over a larger area and volume, so that less laser intensity is needed for implosion. However, a problem exists in producing hollow fuel pellets, ideally hollow spheres of the fuel material, which are of consistently uniform size, shape and thickness. Relatively uniform droplets of liquids have been produced by forcing a liquid out of a nozzle under pressure and using acoustical vibration to obtain uniformity of the droplets (see Schneider and Hendricks, Review of Scientific Instruments, 35, 1349 (1964)). Hollow droplets have also been produced by ultrasonically nucleating bubbles in superheated liquid drops of hydrogen, as disclosed in Foster, Hendricks, and Turnball, Applied Physics Letters, 26, 580 (1975).
It is an object of the present invention to provide a novel method for producing improved relatively uniform hollow spheres of a substance which can be utilized as targets for laser fusion, or utilized for other purposes.